The present invention relates to organic electroluminescent devices and more particularly to doping a hole-transport layer or an electron-transport layer with suitable stabilizing dopants such as rubrene, distant from the emission layer, to improve the operating lifetime of these devices without affecting the electroluminescent color.
A common form of organic electroluminescent device, or organic light-emitting diode (OLED), includes a substrate and, disposed thereupon, a multilayer structure comprising an anode, an optional hole-injection layer, a hole-transport layer, an emission layer, an optional electron-transport layer, and a cathode, each layer of the multilayer structure except the anode and cathode comprising one or more organic or organometallic compounds. Electroluminescent (EL) devices are attractive because of their low driving voltage, high luminance, wide viewing angle, and capability for full-color flat-panel emissive displays. Tang et al. described this OLED structure in their U.S. Pat. Nos. 4,769,292 and 4,885,21 1. With the prospect of using OLEDs in display screens for laptop computers, digital personal organizers, cellular telephones etc., it is highly desirable to improve the operating lifetime of these devices.
The following patents and publications disclose the preparation of OLEDs with improved operating lifetimes. Modifications of the multilayer structure, stabilized cathode materials, and confinement of the charge carriers and their recombination within the emission zone have achieved significant improvement in the operating lifetimes of these devices. So et al., in U.S. Pat. No. 5,853,905, discussed an EL device consisting of a single organic emission layer containing a mixture of electron-transport and hole-transport materials, sandwiched between anode and cathode. However, this device has low efficiency. Popovic et al., in SPIE Conference Proceedings, Vol. 3476, pp. 68-72, 1998, described an EL device with improved efficiency and operating lifetime prepared by mixing an emitting electron-transport material and a hole-transport material in a light-emitting layer. Xie et al., in U.S. Pat. No. 5,989,737, disclosed an OLED in which the hole-transport layer comprises a tertiary aromatic amine doped with a polycyclic aromatic hydrocarbon such as rubrene. This approach has the problem that some dopants, such as rubrene, alter the electroluminescent color of the device.
It is therefore an object of the present invention to provide an OLED with improved operating lifetime.
This object is achieved by an organic electroluminescent device comprising:
a) a substrate;
b) an anode and a cathode positioned relative to the substrate;
c) an emission layer disposed between the anode and the cathode;
d) a hole-transport layer disposed between the anode and the emission layer;
e) an electron-transport layer disposed between the cathode and the emission layer;
f) the hole-transport layer including two or more sublayers, one sublayer containing a stabilizing dopant, and the sublayer adjacent to the emission layer containing no stabilizing dopant; and
g) the stabilizing dopant including rubrene or a derivative thereof.
This object is also achieved by an organic electroluminescent device comprising:
a) a substrate;
b) an anode and a cathode positioned relative to the substrate;
c) an emission layer disposed between the anode and the cathode;
d) a hole-transport layer disposed between the anode and the emission layer;
e) an electron-transport layer disposed between the cathode and the emission layer;
f) the electron-transport layer including two or more sublayers, one sublayer containing a stabilizing dopant, and the sublayer adjacent to the emission layer containing no stabilizing dopant; and
g) the stabilizing dopant including rubrene or a derivative thereof.
This object is further achieved by an organic electroluminescent device comprising:
a) a substrate;
b) an anode and a cathode positioned relative to the substrate;
c) an emission layer disposed between the anode and the cathode;
d) a hole-transport layer disposed between the anode and the emission layer;
e) an electron-transport layer disposed between the cathode and the emission layer;
f) the electron-transport layer including two or more sublayers, one sublayer containing a stabilizing dopant, and the sublayer adjacent to the emission layer containing no stabilizing dopant;
g) the hole-transport layer including two or more sublayers, one sublayer containing a stabilizing dopant, and the sublayer adjacent to the emission layer containing no stabilizing dopant; and
h) the stabilizing dopant or dopants including rubrene or a derivative thereof.
The operating lifetime of the OLED is improved by inclusion of the stabilizing dopant or dopants, while the electroluminescent color and efficiency are substantially unaffected.
The use of a stabilizing dopant distant from the emission layer does not affect the electroluminescent properties of the present OLEDs and permits the electroluminescent color to be tuned solely by adjusting the composition of the emission layer. It has been found that rubrene and derivatives thereof can be used as stabilizing dopants in the hole-transport layer, the electron-transport layer, or both the electron-transport layer and the hole-transport layer of an OLED and that the resulting device has a greater operating lifetime than an otherwise identical device that contains no stabilizing dopant.
Other features and advantages of the present invention include the following:
OLEDs with various electroluminescent colors, such as red, green, blue, or white, can be constructed by inclusion of suitable dopants in the emission layer. Thereby, it is possible to construct OLEDs with simultaneously optimized lifetime and chromaticity;
OLEDs of this invention have high luminance efficiency;
OLEDs of this invention can be used with a low-voltage current source;
OLEDs of the present invention can include a stabilizing dopant in both the hole-transport and the electron-transport layers, in order to provide yet greater operating lifetimes than that of an OLED containing a stabilizing dopant in either the hole-transport layer alone or the electron-transport layer alone; and
OLEDs made in accordance with the present invention have long operating lifetimes and excellent luminance properties.